First instance body dealing with electoral disputes

Constitution

Article 1.

Section 5

Each House shall bethe Judge of the Elections, Returns and Qualifications of its own Members […]

https://www.constituteproject.org/constitution/United_States_of_America_1992.pdf

ElectoralJustice: The International IDEA Handbook

Theclassification is based on the legal nature of the body competent to make thefinal ruling on the last challenge provided for against the election results, evenif it is almost never used.  The competentorgan to make the final ruling on national legislative elections in the UnitedStates is the House of Representatives or the Senate for the election of theirown members, or the whole of Congress for presidential elections; a concurrentmajority vote of both the House of Representatives and the Senate is requiredin order to overturn a decision on this matter reached by a state. This has nothappened since the 1876 presidential election. The most that has happened isthat a challenge has been brought before the Supreme Court. In the case ofnational legislative elections, there have been very few cases brought beforethe House of Representatives or the Senate. Nonetheless, in the light of thecriterion adopted,  the  US EDRS  falls  into the  category  of those  conferred  on  a legislativeorgan, and not among those entrusted to a judicial body.